Convert `struct FromBytesWithNulError` into enum

This PR renames the former `kind` enum from `FromBytesWithNulErrorKind` to `FromBytesWithNulError`, and removes the original struct.

See rust-lang/libs-team#493

## Possible Changes - TBD
* [x] should the new `enum FromBytesWithNulError` derive `Copy`?
* [ ] should there be any new/changed attributes?
* [x] add some more tests

## Problem

One of `CStr` constructors, `CStr::from_bytes_with_nul(bytes: &[u8])` handles 3 cases:
1. `bytes` has one NULL as the last value - creates CStr
2. `bytes` has no NULL - error
3. `bytes` has a NULL in some other position - error

The 3rd case is error that may require lossy conversion, but the 2nd case can easily be handled by the user code. Unfortunately, this function returns an opaque `FromBytesWithNulError` error in both 2nd and 3rd case, so the user cannot detect just the 2nd case - having to re-implement the entire function and bring in the `memchr` dependency.

## Motivating examples or use cases

In [this code](https://github.com/gquintard/varnish-rs/blob/f86d7a87683b08d2e634d63e77d9dc1d24ed4a13/varnish-sys/src/vcl/ws.rs#L158), my FFI code needs to copy user's `&[u8]` into a C-allocated memory blob in a NUL-terminated `CStr` format.  My code must first validate if `&[u8]` has a trailing NUL (case 1), no NUL (adds one on the fly - case 2), or NUL in the middle (3rd case - error). I had to re-implement `from_bytes_with_nul` and add `memchr`dependency just to handle the 2nd case.

r? `@Amanieu`

Rollup of 7 pull requests

Successful merges:

 - rust-lang#132397 (Make missing_abi lint warn-by-default.)
 - rust-lang#133807 (ci: Enable opt-dist for dist-aarch64-linux builds)
 - rust-lang#134143 (Convert `struct FromBytesWithNulError` into enum)
 - rust-lang#134338 (Use a C-safe return type for `__rust_[ui]128_*` overflowing intrinsics)
 - rust-lang#134678 (Update `ReadDir::next` in `std::sys::pal::unix::fs` to use `&raw const (*p).field` instead of `p.byte_offset().cast()`)
 - rust-lang#135424 (Detect unstable lint docs that dont enable their feature)
 - rust-lang#135520 (Make sure we actually use the right trivial lifetime substs when eagerly monomorphizing drop for ADTs)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#134143 - nyurik:err-nul, r=dtolnay

Convert `struct FromBytesWithNulError` into enum

This PR renames the former `kind` enum from `FromBytesWithNulErrorKind` to `FromBytesWithNulError`, and removes the original struct.

See rust-lang/libs-team#493

## Possible Changes - TBD
* [x] should the new `enum FromBytesWithNulError` derive `Copy`?
* [ ] should there be any new/changed attributes?
* [x] add some more tests

## Problem

One of `CStr` constructors, `CStr::from_bytes_with_nul(bytes: &[u8])` handles 3 cases:
1. `bytes` has one NULL as the last value - creates CStr
2. `bytes` has no NULL - error
3. `bytes` has a NULL in some other position - error

The 3rd case is error that may require lossy conversion, but the 2nd case can easily be handled by the user code. Unfortunately, this function returns an opaque `FromBytesWithNulError` error in both 2nd and 3rd case, so the user cannot detect just the 2nd case - having to re-implement the entire function and bring in the `memchr` dependency.

## Motivating examples or use cases

In [this code](https://github.com/gquintard/varnish-rs/blob/f86d7a87683b08d2e634d63e77d9dc1d24ed4a13/varnish-sys/src/vcl/ws.rs#L158), my FFI code needs to copy user's `&[u8]` into a C-allocated memory blob in a NUL-terminated `CStr` format.  My code must first validate if `&[u8]` has a trailing NUL (case 1), no NUL (adds one on the fly - case 2), or NUL in the middle (3rd case - error). I had to re-implement `from_bytes_with_nul` and add `memchr`dependency just to handle the 2nd case.

r? `@Amanieu`